The purpose of this prospective study was to elucidate the characteristics of adult patients with acute abdomen admitted to a Finnish university hospital. A total of 639 patients entered the study. The most common cause of acute abdomen was non-specific abdominal pain (NSAP) (33.0%) followed by acute appendicitis (23.3%) and acute biliary disease (8.8%). The male:female ratio was 47:53 in the whole study population, but there were differences in the age and sex distributions when studied by diagnosis. NSAP was mainly presented in young women, but also in patients over 75 years of age. Acute appendicitis was most frequently found in young men, and biliary disease was most common in elderly women. A male predominance was noted in cases of alcoholic pancreatitis and gastritis, renal stones and peptic ulcer. Young women and elderly patients were slightly over-represented in the study population when compared with the whole population residing within the study area. 43% of the patients were operated on, and surgery was most common in patients aged 55-64 years and 15-24 years. Altogether 15% of the patients were discharged without hospitalisation. Twelve patients (1.9%) died of various causes, the most common of which was malignancy (four patients). In conclusion, a considerably large proportion (one third) of cases with acute abdomen remained without any specific explanation. Further, age and gender seem to be important factors when the most probable cause of acute abdomen is to be considered. Operative treatment is necessary in almost half of cases, and mortality for acute abdomen is low.